1. Field of the Invention
The present invention relates to a polishing apparatus for polishing and finishing a thin-plate-like object such as a semiconductor wafer or a glass plate for an LCD so that its surface will be made flat and uniform in height.
2. Description of the Related Art
FIGS. 1 and 2 show conventional polishing apparatuses for polishing a thin-plate-like object such as a wafer. The polishing apparatus 1 shown in FIG. 1 is composed of a table 3 that supports a wafer 2 mounted thereon and can rotationally drive it, a polishing wheel 5 supported by a spindle 4, and other parts. The table 3 can be rotated in the horizontal plane by an X-axis slide 6 incorporating a rotational driving section as well as moved in the X-axis direction by an X-axis slide mechanism section 7.
The X-axis slide mechanism section 7 is composed of an X-axis ball-thread nut 8 that is fixed to the X-axis slide 6, an X-axis ball-thread screw 9 that is threadedly engaged with the X-axis ball-thread nut 8, an X-axis servo motor 10 for driving the X-axis ball screw 9, and other parts. On the other hand, the polishing wheel 5 is opposed to the table 3 and rotated in the horizontal plane. A Z-axis slide 12 is fixed to the spindle 4 and supported by a Z-axis guide so as to be slidable in the Z-axis direction.
With the above configuration, the wafer 2 that is mounted on the table 3 is polished by rotating the table 3 and the polishing wheel 5 while pressing the polishing wheel 5 against the wafer 2. The entire surface of the wafer 2 is polishing by moving the table 3 in the X-direction with the X-axis slide mechanism section 7.
On the other hand, in the polishing apparatus 1a shown in FIG. 2, a pad 15 is fixed to the top surface of a surface table (rotary table) 14 that is rotationally driven and a wafer 2 is absorbed and fixed on a polishing head 16 via an absorption film 17. The wafer 2 is polished by rotating it while pressing it against the pad 15 by means of a cylinder 18. To prevent scattering of an abrasive 20 or a polishing powder, the wafer 2 is enclosed by a retainer ring 19. A pad conditioner 21 for correcting roughening of the pad 15 is pressed against the pad 15. The wafer 2 is polished and finished by rotating the pad 15 together with the surface table 14 and rotating the wafer 2.
As shown in FIG. 3(A), the surface of a wafer 2 is not flat and is formed with many protrusions 22. To make the surface of the wafer 2 flat and uniform in height, it is necessary to flatten the protrusions 22. Conventionally, a static polishing/planarization method shown in FIG. 3(B) and a dynamic polishing/planarization method shown in FIG. 3(C) are employed as measures for removing the protrusions 22. In the former method, the protrusions 22 are removed by using a polishing pad 23 formed by bonding soft and hard pads together. In the latter method, the protrusions 22 are removed by adjusting the pressing force exerted on the protrusions 22 by utilizing the frequency characteristic of an elastic tool. The object of the polishing/planarization (CMP) is to flatten and uniformize the height of the surface of the wafer 2. In general, the flatness is represented by the difference A-B between height A of the highest point and height B of the lowest point of the protrusions 22 as measured from the base surface. On the other hand, the height uniformity is a percent value obtained by dividing dispersion of the polishing amount in the wafer 2 by an average polishing amount over the entire surface. As shown in FIG. 4, polishing/planarization is performed by removing the protrusions 22 while leaving large undulation of the entire surface of the wafer 2.
FIGS. 5(A)-5(D) show typical ones of various shapes of polishing subject surfaces of wafers 2. FIGS. 5(A)-5(D) show a taper type, a sphere type, a chips type, and a twist type, respectively. Naturally, there are composite types of those.
To polish the surface of a wafer 2, a polishing wheel or the like is pressed against the surface of the wafer 2. In this case, uniform polishing can be attained more easily when the pressing force is constant.
When the surface of a wafer 2 is not flat as in the cases of FIGS. 5(A)-5(D), there occurs an action that the polishing wheel is moved vertically. In this case, the polishing amount is approximately in proportion to a push-up length or a push-down length.
The push-up length and the push-down length are relatively short in the cases of FIGS. 5(B)-5(D). On the other hand, in the case of FIG. 5(A), the push-up length in a central portion is much different from that in a peripheral portion. This causes a difference between the degree of removal of the protrusions 22 in the central portion of the wafer 2 and that in the peripheral portion. In this case, it is particularly difficult to obtain a polished surface that is flat and uniform in height.
On the other hand, as shown in FIG. 7, a deviation in parallelism exists between a polishing axis 24 and a pad axis 25 and there is a squareness deviation of about 1/2,000 in the circumferential direction. Therefore, a wafer 2 and a pad 15 do not contact each other uniformly, which is one factor of lowering the height uniformity in the surface of the wafer 2. Further, in this state, the flatness of the pad 15 is lowered faster and hence the pad 15 needs to be replaced more frequently.
For example, Japanese Unexamined Patent Publication Nos. Hei. 10-29153 and Hei. 10-73420 disclose polishing apparatuses for polishing a wafer to obtain a flat surface. In the publication No. Hei. 10-29153 entitled "Semiconductor Wafer Polishing Apparatus," an air room is provided behind a wafer holding plate for holding a semiconductor wafer and prescribed pressing force is applied to the wafer holding plate uniformly by controlling the inner pressure of the air room. In the publication No. Hei. 10-73420 entitled "Surface Shape Measuring Apparatus and Polishing Apparatus Using It," an object is polished in such a manner that its inclination is adjusted while its surface shape with respect to a reference glass plate is detected.
Although the above conventional techniques are effective for planarization of a wafer, the former technique is macroscopic and cannot obtain a sufficient level of flatness for a wafer having protrusions of different heights at many locations. Although the latter technique employs a piezoelectric element as a surface adjusting means, it cannot obtain a sufficient level of flatness for a wafer having a surface shape as shown in FIG. 5(A). Further, since the adjusting means of this technique is not so specific and close as that of the invention as described in this specification, this technique cannot provide sufficient levels of flatness and height uniformity.